Note that the spread in ice levels in March is about one and a half million sq km (15 to 13.5). The spread in mid-September is about four million sq km. The winter variation is much smaller than the summer variation. IOTW, the winter ice extent, although showing a small down trend, is always (roughly) 14 million sq km. The summer extent has been dropping from 7.5 to 3.5 million sq km.

By asymmetrical, I meant the summer variation is much more sensitive than the winter. This is why summer ice extents are such a useful indicator of warming.

The effect is even more dramatic when you plot ice volumes instead of areas, although volume data depends partly on models, while area is a direct measurement from a satellite. The latter is not open to dispute, while the models are always being attacked as biased.

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